Flow-through and cyclic fed-batch cultivation of the green microalga Haematococcus pluvialis in recirculating aquaculture effluent: Strategies for sustained biomass production and astaxanthin biosynthesis

Integrating microalgal cultivation with recirculating aquaculture systems (RAS) offers a sustainable solution for nutrient recovery and the production of high-value bioproducts. The conventional batch cultivation technique for microalgae is constrained by limited biomass yields and short cultivation cycles. This study investigated the potential of cultivating Haematococcus pluvialis in RAS effluent and assessed flow-through and cyclic fed-batch cultivation strategies to enhance biomass and astaxanthin production. Both cultivation modes were investigated in two separate laboratory-scale experiments. Experiment I used (i) synthetic medium (modified BG-11) and (ii) untreated RAS effluent, while Experiment II used (i) autoclaved and (ii) filtered-autoclaved RAS effluent. Light stress was applied to the outflow from flow-through and the withdrawn culture from cyclic fed-batch cultivation to stimulate astaxanthin biosynthesis. In both experiments, the flow-through mode consistently outperformed the cyclic fed-batch mode in growth rate, cell density, biomass concentration, and nutrient uptake. In Experiment I, untreated RAS effluent supported H. pluvialis growth; however, microbial interference during the post-growth phase affected astaxanthin synthesis. In Experiment II, using pretreated RAS effluent, the flow-through mode achieved a maximum growth rate of 0.39 ± 0.01 day⁻¹ , nitrate removal of 64.28 ± 1.84 % and ∼100 % phosphate removal. The cyclic fed-batch mode achieved a maximum growth rate of 0.34 ± 0.01 day⁻¹ , nitrate removal of 55.05 ± 6.25 % and ∼100 % phosphate removal. Although, astaxanthin content was similar between modes (6.30 ± 0.48 mgg⁻¹ DW), the higher biomass concentration in flow-through mode resulted in a higher astaxanthin concentration (2.96 ± 0.78 mgL⁻¹) compared to cyclic fed-batch mode (1.6 ± 0.31 mgL⁻¹).